The Australian Federal Police Commissioner, Mick Keelty, yesterday strongly denied his team had used any tracking devices on the vessel known as SIEV X, which sank in October last year drowning 353 asylum seekers.

However, Mr Keelty is still refusing to answer questions from Labor senators at Wednesday night's Senate estimates hearing on whether the force has used tracking devices as a part of its program to disrupt people-smuggling, claiming "public interest immunity".

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He stuck by his comments from July that the federal police had no means of surveillance of the distressed SIEV X vessel, other than "human sources".

"SIEV X was not tracked by any one in the AFP by any means whatsoever and, in fact, we do not know where the SIEV X sank," he said.

When asked by Labor's home affairs spokesman, John Faulkner, earlier this week whether listening or tracking devices were used, Mr Keelty said he had to contact his operational officers, who were in Bali, to get an answer.

He said he had no understanding of whether tracking devices were used.

But Mr Kelty did say: "My initial response is that we would not have been involved in listening devices because I do not think it is permitted under the legislation."

During the hearing, the Justice Minister, Chris Ellison, cut off further questions about Mr Keelty's knowledge of the use of tracking devices.

When asked about the practice, Senator Ellison said: "Offhand, I am not aware of anything in relation to a tracking or listening device - operational details are not conveyed to me."

However, the next day, on Thursday, Mr Keelty sent a letter to the estimates committee saying that he had considered the questions he had taken on notice and had decided to claim "public interest immunity", on the grounds the answers would disclose the AFP's lawful detection methods and prejudice their effectiveness.

"As far as we are concerned it is a legitimate claims for public interest immunity," Mr Keelty said.

Asked why he had disclosed that listening devices were not used, he claimed Senator Faulkner had been using the questions about listening devices "as a disguise to try and find out whether we used tracking device".